Tire engineers have historically attempted to build very durable casing structures that can survive the severe driving conditions that the vehicle operators put the tires through.
Early on tires were very heavy and employed many layers or plies of bias cord. The primary objective was to simply retain the air and avoid a flat or deflation.
Through a process of endless research to develop more durable and better tire constructions new materials and better designs have been developed.
The introduction of the radial tire made it practical to develop tires having as few as one carcass ply. The ply was contained radially by a belt structure. To enhance the durability of the tire these belt structures evolved to become primarily steel reinforced. These steel reinforced belts yielded and currently provide a very durable structure.
These steel belted tires have many benefits that make their use attractive. The steel cords are not heat sensitive that is their physical properties are pretty much constant regardless of the tire's operating temperature. The steel cords are substantially inextensible and the cords can be made very high strength with fine filaments that have excellent fatigue resistance. Nevertheless these steel corded belts in tires have resulted in the need to add rubber gauge directly above the belts in the area commonly referred to as the undertread, in the belt layers themselves and in the areas of the belt edges, all in an attempt to keep these steel cords from becoming exposed or structurally separating at the edges. Furthermore the aramid reinforced cords have a high tenacity making them effectively lighter in weight for the same strength as the steel cords used in belts.
The resultant effect has been that the steel belted radial tires are in fact heavier using more rubber in the tread area and in the tire's shoulders. It is precisely in these areas that a large part of the tire's tread wear performance and sensitivity to rolling resistance must be the highest. The more rubber in this area the higher the hysertesis effects and higher the temperatures under running conditions.
It is now an object of tire designers to develop tires that generate lower car fuel consumption. This can be achieved by designing cool running tires that have low mass and low rotational inertia while increasing the tire's handling performance and treadwear, furthermore, the engineer must insure that the tire's footprint and the contact patch of the tread has a uniform pressure distribution in order to achieve uniform wear.
With the advent of high performance tires having very low aspect ratios the use of belt structures having overlays of synthetic cords of nylon or aramid has been common. To further achieve high speed performance the tread thickness has been kept at a minimum. Thick tread mass at high speeds simply want to fly off the tire. As these tires are pushed to the engineer's known tire design limit he must rethink the entire parameters of the tire. In some cases this means going back and reanalyzing the concepts that were used in the past but were abandoned as a result of those in the art pursuing a different path.
One such approach that heretofore had favor among tire engineers was the use of aramid folded belts which although a very good material for belts has lost favor to lower cost steel belts. The primary downfall of folded aramid belts was their apparent lack of adhesion durability and the high cost.
In U.S. Pat. No. 5,332,018 issued Jul. 26, 1994, to Roesgen et al., a novel folded belt assembly having at least two plies, with at least one of the belt plies having at least one folded portion such that there is at least one folded portion of a belt ply one on each side of the belt assembly. A spirally wound strip of a cord reinforced elastomer extends transversely between the folded portions. The resultant structure, although difficult to assemble due to the folds, provided a tire which is durable at high speeds and which at the same time has a high degree of uniformity. The use of folded belt layer complicated the manufacture and inherently added some cost and weight penalties.
The tire, according to the present invention, eliminates the use of folded belts when using a plurality of non-folded layers of aramid cord reinforced elastomers. The resultant structure is very durable without having the adhesion problems common with aramid cords while at the same time being much lighter than conventional belts reinforced with steel cords.